Table of contents

Odd Rydberg states of lithium in a uniform magnetic field have been excited from the ground state by sigma polarised light. In the recorded energy range, as the field strength increases, several adjacent diamagnetic n manifolds overlap and narrow anticrossings are observed. In spite of the small value of the quantum defect involved ( delta _p=0.053), the core effects are responsible for the major part of the recorded anticrossing widths.

Cross sections for optically allowed transitions in positive ions, produced by electron impact, have contributions from an inner region for which the colliding electron penetrates the target, and from an outer region for which it does not. The contributions from the outer region converge rather slowly but can be summed using a method due to Burgess (1974) based on the Coulomb-Bethe approximation. A discussion is given on the use of the method in conjunction with close-coupling calculations. Results are presented for three transitions in C²⁺ using close coupling both with and without exchange.

The time evolution of level population and photon number operators is found in the rotating wave approximation.

Electron correlation has been examined in detail within the K alpha K beta , K alpha L alpha and K beta L alpha shells in position space for a series of Li-like systems in their ground state (1s²2s alpha )²S. This necessitated the partitioning of the two-particle space-spin density and was straightforward for the Hartree-Fock description. For the correlated density, a previously devised partitioning technique based on a many-electron representation of a CI wavefunction was employed. Using existing wavefunctions, Coulomb holes, partial Coulomb holes and several one- and two-particle expectation values, and their percentage changes due to correlation, were evaluated for the intra- and inter-electronic shells. Since 3<or=Z<or=8, it was possible to examine the Z-dependent trends of the radial and angular components of electron correlation for each shell. The presence or otherwise of Fermi correlation in the inter-shell descriptions had a marked influence on their Coulomb correlation characteristics. The findings for the Li-like intra- and inter-shells were compared both with a previous analysis of the He-like and Be-like series of ions and also with a similar examination of the 2¹S and 2³S excited states of He.

Laser-atomic-beam spectroscopy was performed to determine precise values of the scalar and tensor polarisabilities of the Sm I 5720.19 AA line in order to get a sub-standard for a calibration of Stark-effect fields. The values obtained were alpha ₀=-91.8(4) and alpha ₂=13.29(6) kHz kV^-2 cm². A comparison with theoretical results shows good agreement.

Perturbation theory for the interaction of light with a binary system is applied to collisional redistribution of radiation in the weak-field limit. Within the very large domain of validity of the binary model applied to collisions between non-hydrogenic atoms and atomic perturbers at low densities, redistribution can be viewed as collisional-induced absorption followed by spontaneous decay during a second collision. The formalism can be applied from the quasimolecular domain at large detunings Delta omega ₁ or Delta omega ₂ up to the impact regime, but emphasis is placed here on the non-impact region, at small Delta omega ₁ or Delta omega ₂, where non-adiabatic effects are important in collision-induced radiative absorption or emission rates as well as in the redistribution. The impact limit of the redistribution function is easily rederived in this way.

The influence of the pulsed shape and/or chirp on multipeak structures that appear in both photoelectron spectra in laser-induced autoionisation and in near-resonant Raman-scattering photon spectra is investigated. The analytic results are presented for both types of spectra for a wide variety of smooth strong pulses. Possible experimental verification of the multipeak structures in the spectra is also discussed.

Using a DC discharge VUV source equipped with a rotating, focusing reflection polariser in conjunction with a hemispherical electron spectrometer, the angular distribution of photoelectrons from the outermost p levels in the noble gases Ar, Kr and Xe was measured in the kinetic-energy region 0-3 eV. Discharges in Ne, Ar, Kr, Xe and N₂ are used to obtain photons with energies in the range 11-19 eV. For photon energies between the ionisation thresholds for the P_3/2 and P_1/2 levels, observed rapid oscillations in beta are correlated with resonances with autoionising states. For higher photon energies, the energy dependence of beta is smooth.

The photoabsorption of the rare-gas excimers Ne₂*, Ar₂*, and Kr₂* for wavelengths between 300 and 520 nm is investigated. Rydberg series are found for all three molecules and are attributed to Rg₂*(ms ³ Sigma _u⁺(0_u^-,1_u)) to Rg₂*(np ³ Pi _g(1_g, 2_g)) transitions. The molecular ionisation energies are (34340+or-60), (29345+or-25) and (28490+or-40) cm^-1 for Ne₂*, Ar₂* and Kr₂* respectively. The molecular quantum defects are found to be slightly smaller than the defects of the corresponding asymptotic atomic states. The dissociation energies of the Rg₂*(m³ Sigma _u⁺) states are (5421+or-460), (5643+or-140) and (4823+or-60) cm^-1 for Ne₂*, Ar₂* and Kr₂* respectively.

The absorption profiles of the far wings of potassium 4S to 3D and 4S to 5P lines are experimentally investigated. From the analysis of these profiles the authors obtain information about the excited-state potentials. The results are compared with both calculated pseudo-potentials and model potentials.

The proton-induced 3s-3p excitation of spin-polarised Na atoms has been studied both experimentally and theoretically. The total excitation cross section as well as the Stokes' polarisation parameters of the fluorescence light were measured in the regime of intermediate impact energies (0.8-6 keV). The measured circular polarisation P₃ is in accordance with conservation of spin orientation during the collision. The linear polarisation P₁ reflects the collisional induced alignment of the 3p state. The measured total cross section and the alignment are compared with various calculations. The p+Na collision system can be treated as a single active outer electron in the field of two inert cores.

The K-shell ionisation cross sections sigma _KI have been measured for a Gd target at incident deuteron energies of 0.9-2.8 MeV. The absolute sigma _KI values have been determined by normalisation to the known Coulomb excitation cross section sigma _C of the target nucleus. The experimental results are compared with the theoretical predictions of the semiclassical approximation (SCA) and the plane-wave Born approximation (PWBA) models including electron binding, Coulomb repulsion and relativistic corrections. The sigma _He/4 sigma _D and sigma _H/ sigma _D ratios of the cross sections have also been compared with the SCA model predictions by using the earlier results for protons and alpha particles.

The total cross section for single electron capture from He by Li³⁺ ions has been calculated using the close-coupling technique in the impact energy range 1.5*10^-4-43 keV. The model-potential method was used to determine the electronic energy curves of the (LiHe)³⁺ molecule. The dynamic couplings between the molecular states were obtained by numerical differentiation of the model-Hamiltonian stationary wavefunctions. Depending on the impact energy, the collision dynamics were solved either by a full quantum-mechanical treatment or using the semiclassical impact parameter approximation. The total charge exchange cross section is in good agreement with the experimental results of Wirkner-Bott et al. (1981).

Characteristic energies of low-energy electrons in krypton and xenon gases have been measured at room temperature by the Townsend method as a function of reduced electric field. For both gases, observed characteristic energies were twice or even three times as large as the early estimates by Frost and Phelps (1964) based on the momentum transfer cross sections for electrons in these gases derived from drift velocity data. The cross sections were determined from the present experimental data over an energy range from 0.01 to 6.0 eV for krypton and from 0.01 to 5.0 eV for xenon. The minimum values and positions of Ramsauer minima in the derived cross sections were 1.0*10^-17 cm² at 0.5 eV for krypton and 4.0*10^-17 cm² at 0.6 eV for xenon. These minimum values are both definitely smaller than the previous experimental results of Frost and Phelps, while being rather close to, but still somewhat smaller than, those of Hoffmann and Skarsgard (1969). Theoretical cross sections calculated for krypton and xenon by Sin Fai Lam (1982) and by McEachran and Stauffer (1984) and those for krypton by Fon et al. (1984) all agree with the present experimental ones within +or-30% for energies below about 5 eV in both gases.

The Bethe approximation to the reactance matrix is considered for electron-neutral-atom collisions. Analytic expressions are given for the matrix elements. For the special case of electron-neutral-atom scattering the sum rules of Burgess (1974) are simplified. Particular consideration is given to the problem of calculating cross sections for dipole transitions. Partial cross sections are presented for all non-exact resonance dipole transitions between hydrogen atom states, with n, n'<or=4, for a range of energies with l>or=11, 31, 51, 71, 91.

Born partial-wave integrals have been performed for a range of energies and total cross sections are presented for electron-hydrogen-atom excitation both in the conventional first Born approximation (Born I) and the unitarised Born approximation (Born II). Exchange effects are not included. With the exception of exact resonance-allowed transitions (n=n', lambda =1) results are presented for all transitions between hydrogen-atom states whose principal quantum number is less than or equal to 4.

Differential electron impact cross sections (DCS) for the elastic scattering and excitation of the ¹ Pi and 2¹ Sigma ⁺ states of N₂O have been measured using a crossed electron beam-molecular beam scattering technique. These measurements were performed at electron impact energies of 10 and 12 eV for elastic as well as at 15, 20, 30, 50 and 80 eV for elastic and inelastic scattering. DCS were obtained over a wide range of scattering angles, up to 150 degrees . The elastic DCS were put on an absolute scale by normalisation to the total electron scattering cross sections. The inelastic DCS are in relative units except for the case of 80 eV impact energy where it was possible to determine the elastic to inelastic intensity ratio and to obtain absolute DCS values. By extrapolation of DCS to zero and 180 degrees scattering angles, integral and momentum transfer cross sections were obtained.

Full details are given of an experiment in which the three normalised Stokes' parameters characterising the decay of the electron impact excited d ³ Pi _u (v₁=0, N₁=1) rotational level of H₂ were determined in an electron-photon coincidence experiment at 25 eV impact energy and over the angular range 20-60 degrees . An analysis shows how these parameters are related to the state multipoles characterising the excited state. The rather low degree of coherence which was observed is consistent with various depolarising effects, namely the averaging over magnetic sublevels which occurred in both the excitation and decay channels and also the perturbations due to fine and hyperfine structure relaxations.

Electron attachment cross sections are reported in the electron energy range 0.160 meV and at energy resolutions of 4.5-7.5 meV (FWHM) for the molecules c-C₆F₁₀ (perfluorocyclohexene), c-C₆F₁₂ (perfluoro-1,2-dimethylcyclobutane), C₈F₁₆ (perfluoro-1,3-dimethylcyclohexane) and 1,1,2-C₂Cl₃F₃ (1,1,2-trichlorotrifluoroethane). Use is made of the Kr photoionisation technique, and measured attachment lineshapes are converted to cross sections by normalisation through attachment rate constants. Comparisons are made with attachment cross sections derived from swarm-measured rate constants. Similar to previous results in eight other molecules, the present four molecules exhibit resolution-limited onsets at threshold consistent with an s-wave attachment behaviour for which sigma _A( epsilon ) approximately epsilon ^-12/, and with a neutral-negative-ion curve crossing at zero energy.

Presents an explicit calculation of the dielectronic recombination rate coefficients for eight helium-like ions ranging from neon to xenon using relativistic multiconfiguration wavefunctions to calculate both the Auger and radiative matrix elements. The behaviour of the partial dielectronic recombination rate coefficients, via the 2lnl' manifolds, for the various principal quantum numbers n is examined across the periodic table.